Many prior art patents have disclosed using metallic salts as photolyzing agent for photo-degradable plastics. Examples of these patents include U.S. Pat. Nos. 3,840,512, 3,865,767, 3,882,058, in which metallic salts, including those of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, of fatty acids have been disclosed as effective photolyzing agents, or the so-called photo-prodegradant, to initiate and/or accelerate a photo-degradation of various plastics. The contents of these prior art patents are herein incorporated by reference.
In U.S. Pat. No. 3,865,767 (the '767 patent), it is disclosed a "di-soap" type photolyzing agent represented by the following formula: ##STR1## wherein Y is an aliphatic fatty acid having a carbon number from about 6 to about 25. In the '767 patent, it is demonstrated that when part of ligands attached to the metallic ion are substituted with aliphatic carboxylate (from the fatty acid), the resultant metallic salt achieved good compatibility with plastics and can serve as excellent photo-prodegradant. Other than their applications as photolyzing agents, metallic salts of fatty acids have also been widely used as dry hardening agents for paints and inks, lubricants for plastics and rubbers, hardening accelerators for unsaturated esters, and as stabilizers for plastics.
Presently, there are three main methods for making metallic salts of fatty acids, to wit: (1) melt method, (2) double decomposition method, and (3) exchange method. In the melt method, the fatty acid is heat-melted then directly reacted with oxides or hydroxides of metals to form metallic salts of fatty acids. The melt method requires relatively simple production facility, but has the disadvantages in that: (a) the reaction takes place at elevated temperatures of about 150.degree. C.-200.degree. C., thus often resulting in turning the reaction product into darkened colors; (b) it is difficult to achieve complete reaction and some reactants are likely to remain in the final metallic salt products; (c) the final products must be ground, milled and/or pulverized after the reaction, this complicates the production process and increases the production cost; and (d) metal oxides often contain a variety of metal ions, thus it is often quite difficult to obtain high purity products from the melt method.
The second method, which is the so-called "double decomposition" method, involves primarily a two-step procedure. In the first step, fatty acid is reacted with sodium hydroxide or other alkali hydroxides to form sodium soap or other alkali soaps. This step is the so-called "saponification". In the subsequent or second step, an aqueous metallic salt solution is then added to the sodium soap from the first step to form metallic salts of the corresponding fatty acid. The second method takes place at relatively low reaction temperatures, between about 60.degree. C. to 90.degree. C. The double decomposition method ameliorates many of the disadvantages identified with the melt method disclosed above--the metallic salts obtained therefrom maintain appropriate color, contain high purity of metal, and the reaction products are in a fine powder form. With these advantages, the double decomposition method has become the most commonly used method in the commercial production of metallic fatty acid salts.
With the increased demand on the quality, particularly purity, of the metallic fatty acid salts, however, the conventional double decomposition method no longer appears satisfactory. The problems arise in that the metallic fatty acid salts produced from the double decomposition method do not provide high purity product; they often contain large amounts of unreacted fatty acids and/or sodium or alkali soaps. This problem is illustrated in U.S.S.R. Pat. No. SU 3,795,565 (the SU '565 citation), which teaches the preparation of ferric stearate using the double decomposition method. The SU '565 citation disclosed that the double decomposition method was impossible to achieve complete reaction and that about 7-10% of sodium stearate soap would still remain in the ferric stearate product. Sodium soaps including sodium stearate often cause filtration problems in the final product; therefore, in commercial processes, it is generally necessary to convert sodium stearate back to stearic acid. U.S. Pat. No. 3,840,512 disclosed a synergistic effect between fatty acid and metallic fatty acid salt in the photo-degradation of plastics. This beneficial effect, however, often cannot be realized in actual situations, and the presence of fatty acid is indeed undesirable. Due to their low molecular weight, fatty acid molecules can easily migrate to the surface of plastics matrix in which they are initially uniformly dispersed. This often adversely affects the fabricability (such as printability, sealability, etc) of the plastics which contain these fatty acid salts additives. Also, the migration of the fatty acid molecules causes a nonuniform distribution thereof within the polymer matrix, resulting an uneven or uncontrolled rate of degradation which can be easily affected by the storage environment, storage time, or even different parts of the product. To reduce the amount of residual stearic acid, the double decomposition process is often followed by a rinsing step requiring large amounts of washing solvent. This could result in substantial increase in the production cost and adds the risks of causing serious environmental pollution problems.
The third method, which is called the exchange method, is used primarily for the production of high purity tri-valent or quadruple-valent metallic fatty acid salts. The exchange method involves using excess amounts of fatty acid and the metallic salts of alkylates or alkoxides to effectuate an exchange reaction therebetween in the presence of large amounts of solvent to produce metallic fatty acid salts and alcohols. After the reaction, unreacted residual fatty acid is washed off with solvent. Japanese Pat. Pub. Sho 62-120339 (Sho '339 citation) discloses an example of the process of preparing ferric stearate using such an exchange method. In the Sho '339 citation excessive amounts of fatty acid (stearic acid) and Fe(OCH.sub.3).sub.3 were dissolved in THF to cause an exchange reaction. Ferric stearate was formed as a reaction product. Because this process involves large amounts of THF, it incurs high production cost and causes the accompanying pollution problems. Furthermore, due to the excessive amounts of fatty acid that are used as reactants, about 90% of the fatty acid was unreacted and will have to be discharged along with the solvent. This further increases the cost of the product and reduces its competitiveness.